The term “acoustic device” is understood to mean any structure using acoustic wave propagation and notably acoustic filters or resonators.
Such devices have operating frequencies that may range from a few Hz to a few GHz. The potential applications of these devices may be as acoustic lenses and RF filters, which may be used in wireless communication and notably in mobile telephony, or else as many anti-vibration or RF devices.
According to the known art, as described notably in the article by M. S. Kushwaha, P. Halevi, L. Dobrzynski and B. Djafari-Rouhani, Acoustic Band Structure of Periodic Elastic Composites, Physical Review Letters, Vol. 71 (13), pp. 2022-2025, 1993, phononic crystals are formed by a periodic organization of several, usually two, acoustically different materials. These systems have bandgaps, that is to say frequencies for which no acoustic wave can propagate. These organizations may be in one, two or three dimensions. In one dimension, they correspond to Bragg mirrors, as described in the article by W. E. Newell, Face-mounted piezoelectric resonators, Proceedings of the IEEE, Vol. 53, pp. 575-581, 1965, encountered notably in BAW (Bulk Acoustic Wave) resonators. These structures are widely used in the industry. Two-dimensional or three-dimensional structures are currently still in the research state.
According to the known art, these structures are generally produced by excavating holes in a material, as illustrated in FIG. 1 (a corresponding to the periodicity of the features, h to their height and d to their diameter), and sometimes by partially or completely filling them with another material. The holes produced tend to be as cylindrical as possible, but it is well known that it is not technologically possible to produce perfect cylinders. In fact, the holes tend to be conical. Sarah Benchabane's thesis “Guidage et filtrage des ondes dans les cristaux phononiques [Wave guiding and filtering in phononic crystals]”, U.F.R. des Sciences et Techniques de l'Université de Franche-Comté, 2006, explained that the holes produced for SAW devices are etched with an apex half-angle of about 20° (therefore a 70° slope in the material). This angle leads to surface wave losses in the substrate, and therefore poses a problem. The literature on phononic crystals in thin films does not report the etching angle of the inclusions, but it is generally accepted that this parameter represents a drawback, by analogy with SAW (surface acoustic wave) systems.
One of the important parameters in the field of phononic crystals is the frequency width of the bandgap. It is generally sought to obtain the largest possible bandgaps. It is often possible to widen the bandgap by increasing the diameter of the inclusions, as described in the article by S. Mohammadi, A. A. Eftekhar, A. Khelif, H. Moubchir, R. Westafer, W. D. Hunt and A. Adibi, Complete phononic bandgaps and bandgap maps in two-dimensional silicon phononic crystal plates, Electronic Letters, Vol. 43 (16), pp. 898-899, 2007 (i.e. increasing the parameter d/a, as shown schematically in FIG. 1), but technological limits are rapidly reached. This is because the practical production of devices does not allow dimensions of any size to be produced. Reducing the size leads to complicating the technological steps, and consequently a cost increase.
Another solution for producing AlN phononic crystals has also been proposed by Professor Piazza's team. Specifically, this team proposes AlN inclusions in an air matrix, these inclusions being connected together by “bridges”. This solution remains complex as it requires the thickness of the “bridges” to be properly controlled (N. K. Kuo, C. Zuo and G. Piazza, Microscale inverse acoustic bandgap structure in aluminum nitride, Applied Physics Letters, 95, No. 093501, September 2009; N. K. Kuo, C. Zuo and G. Piazza, Demonstration of inverse acoustic bandgap structures in AlN and integration with piezoelectric contour mode wideband transducers, IEEE, 2009).